Non-bonded hydrogen in a-Si:H

We report deuteron and proton nuclear magnetic resonance (NMR) measurements on high quality plasma enhanced chemical vapor deposition amorphous silicon films containing from 5 to 15 at.% hydrogen (H and/or D). Our results show that from 2% to 40% of the hydrogen in these samples is not atomically bonded by the Si-H chemical bond. We present conclusive evidence that in highest quality films nearly all of this non-bonded hydrogen is present as isolated hydrogen molecules. These molecules are located in centers of atomic dimensions, perhaps in the analogue of T-sites in crystalline Si. In high quality films these molecules are not the small population of densely packed molecules in the occasional microvoids. The photoelectronic quality of the films (as measured by the eta mu tau photoresponse product) increases as the fraction of non-bonded hydrogen increases. Our results are consistent with IR spectral measures of hydrogen. Our experiments also suggest that the non-bonded hydrogen is in the vicinity of light induced defects. Finally by comparing successive annealings and other studies it appears that the molecular hydrogen can be identified with the less-clustered proton narrow NMR line. (C) 1998 Elsevier Science B.V. All rights reserved.